The present invention relates to an electric power supply system, and more particularly to an improvement in the control circuit for shutting down a constant-current supply for a wire telecommunications system operated in a double-end feeding mode.
In a long-haul wire telecommunications system, for example, an underwater cable transmission system, a number of repeaters between the terminal stations are supplied with electric power from constant-current supplies installed at one or both of the terminal stations. These two modes of power supply are referred to as double-end feeding and single-end feeding, respectively. General discussions on the configuration and operation of the constant-current supplies in a double-end feeding system for a wire telecommunications system are given in the following with reference to FIGS. 1 and 2.
FIG. 1 is a conceptual circuit diagram of a double-end feeding power supply system. The system comprises a number of repeaters from 1 to n and constant-current supplies 11 and 12 positioned at both terminal stations. The repeaters are connected in series with a transmission line 50 (e.g., an underwater cable), between the respective output terminals 111 and 121 of the current supplies 11 and 12. The constant-current supplies 11 and 12 include respective constant-current generating sources 13 and 17, resistors 14 and 18, overvoltage protecting elements 15 and 19, and bypassing diodes 16 and 20. The resistors 14 and 18, which are referred to as slope resistors, provide the current-voltage characteristic curve of the sources 13 and 17 with a desired tilt in the constant-current region, as explained later. The overvoltage protecting elements 15 and 19, such as voltage limiters, protect the repeaters from a transient overvoltage caused when the power supply system is open-circuited, for example, and the bypassing diodes 16 and 20 keep the wire telecommunications system operable even when one of the current supplies 11 or 12 becomes inoperable.
In an underwater cable transmission system using fiber optics, hundreds of repeaters are connected in series with a span of approximately 30 km between adjacent repeaters, and a voltage E of, for example, 7000 volts, is fed from each of the constant-current supplies 11 and 12. As shown in FIG. 1, the constant-current supplies 11 and 12 are connected in series in the circuitry of the power supply system. That is, the polarities of the output voltages E of the constant-current supplies 11 and 12 are opposite to each other with respect to ground. Accordingly, the potential at the midpoint of the power supply line 50 is at ground level.
The output voltage E in the double-end feeding system is approximately one-half the voltage necessary when the repeaters are assumed to be fed by a single-end feeding system. This implies that the withstand voltages required for the circuit components and the assemblages thereof for the power supply system including the constant-current supplies, repeaters and the power feeding line, can be reduced to one-half those required in a single-end power feeding system. Thus, the task of designing a power supply system, as well as a reduction in cost, can be facilitated by employing the double-end feeding configuration.
As the distance between the terminal stations increases, the number of repeaters increases, and the feeding voltage E becomes higher, so that the double-end feeding configuration becomes an indispensable technology to a long-haul wire telecommunication system. However, it is important in the double-end feeding configuration that each of the constant-current supplies therefor should not have a power which is as great as the power for a single-end feeding system. In other words, the constant-current supplies should be designed to be applicable only to the double-end feeding configuration. The problem with constant-current supplies which have a large power in a double-end feeding configuration is described below.
FIG. 2 is the current-voltage characteristic diagram of a constant-current supply which is capable of providing the system shown in FIG. 1 with the necessary power, even for a single-end feeding mode. Referring to FIG. 2, the current and voltage supplied by the constant-current supply are regulated along the solid line in accordance with the change in the load resistance connected thereto. The dotted line connecting the origin 0 and the point "a" on the current-voltage curve corresponds to a load characteristic curve when the constant-current supply is operated in a double-end feeding mode, and the dotted line connecting the origin 0 and the point "b" on the current-voltage curve corresponds to a load characteristic curve when the constant-current supply is operated in a single-end feeding mode. Therefore, the points "a" and "b" are the respective stable points for the operations in the two modes. That is, the output voltage of the constant-current supply becomes steady at the voltage Ea in the double-end feeding mode and at Eb in the single-end feeding mode. The current-voltage curve exhibits a slope having a negative tilt in the voltage range between 0 and Ec. The negative tilt is provided by the above-mentioned resistor 14 or 18 so as to reduce the voltage fluctuation during the current regulating operation.
The constant-current supply reveals a so-called drooping characteristic for a voltage higher than Ec. That is, as the voltage increases, the current begins to decrease at the point "c" corresponding to the voltage Ec, and becomes zero at the voltage Ee. Accordingly, in a normal operation, the repeaters are protected from the application of a voltage larger than Ee. However, there may be a fast transient overvoltage higher than Ee, which is caused when the power feeding system line is abruptly cut off, for example. The above-mentioned overvoltage protecting elements 15 and 19 in FIG. 1, having a threshold voltage Ef, are provided to protect the repeaters from a transient overvoltage higher than Ef.
A constant-current supply is generally equipped with a circuit for shutting down the supply of current to the repeaters when the current supply loses its regulation function or when the load resistance on the power feeding line becomes larger than a predetermined value. The shutdown circuit is activated when the output current or voltage of the current supply become larger than a predetermined current Ig or a predetermined voltage Ed, both indicated in FIG. 2. The predetermined voltage Ed is usually selected to be in the voltage range in which the current supply exhibits the above-mentioned drooping characteristic. The current Ig, the voltage Ed and the corresponding point "d" on the drooping characteristic curve are referred to as the shutdown current, shutdown voltage and shutdown point, respectively. It is obvious from FIG. 2 that the above voltages are generally in the relationships, 2Ea=Eb and Eb&lt;Ec&lt;Ed&lt;Ee&lt;Ef.
It should be noted that there is an inevitable timing difference between the starts of the operations of the constant-current supplies in a double-end feeding power supply system. Accordingly, at the beginning of operation, the double-end feeding power supply system is in a pseudo singleend feeding mode, and the output current and voltage of the current supply which starts its operation first, change along the dotted line connecting the origin 0 and the point "b" in FIG. 2. In this constant-current supply, the drooping characteristic does not appear even when the output voltage reaches Eb corresponding to the point "b". Thus, if the double-end feeding power supply system as shown in FIG. 1 is fed by constant-current supplies having characteristics as shown in FIG. 2, the voltage +E or -E may occasionally rise up to Eb because of the starting timing difference in the constant-current supplies.
When both constant-current supplies have begun their operations, the voltages become stable at Ea. The above overvoltage during the pseudo single-end feeding mode requires the repeaters and the power feeding line to withstand a voltage of Eb, at least, which is approximately double the operating voltage Ea in the double-end feeding system. Therefore, such excess power is not only useless but is also undesirable in view of the above-mentioned withstand voltage. Accordingly, the constant-current supplies for a double-end power supply system should be those which have merely enough power for the operation in the double-end feeding mode.
The current-voltage characteristic of a constant-current supply which is designed to be exclusively used in a double-end feeding system is explained together with a conventional shutdown control therefor with reference to FIG. 3, wherein like references designate like or corresponding parts in FIG. 2. Referring to FIG. 3, the dotted line connecting the origin 0 and the point "a" corresponds to the load characteristic curve of the constant-current supply operating in a double-end feeding mode, and the dotted line connecting the origin 0 and the point "b1" corresponds to the load characteristic curve in the above-mentioned pseudo single-end feeding mode at the beginning of the operation of the double-end feeding system.
The constant current supply is provided with a drooping characteristic as represented by the solid line connecting the point "c" and the point Ee1 on the voltage axis as shown in FIG. 3. The drooping point "c" is set adjacent to the stable point "a" in the double-end feeding operation, and corresponds to a drooping voltage Ec which is shifted lower than the voltage Eb and is different from the voltage Ec in FIG. 2. During the period of the pseudo single-end feeding mode, the current and voltage increase along the dotted line connecting the origin 0 and the point "b1" on the drooping characteristic curve. The current and voltage reach a stable point "a" through the points "b1" and "c" when the other constant-current supply begins its operation. Thus, the voltage E in the double-end feeding system can be reduced by using constant-current supplies having the characteristics shown in FIG. 3. As a result, the threshold voltage Ef of the above-mentioned overvoltage protecting elements 15 and 19 in FIG. 1 can be lower, and the withstand voltages for the circuit components and the assemblages thereof of the current supplies and repeaters, can also be decreased.
The constant-current supply having current-voltage characteristics as shown in FIG. 3 is provided with a circuit for shutdown thereof when the current or voltage exceeds the respective predetermined values Ig and Ed. The predetermined voltage Ed is selected to correspond to a point "d" on the drooping charcteristic curve. Obviously, the voltage Ed must be between a voltage Eb1 (not shown) corresponding to the point "b1" and the voltage Ee1. Therefore, a higher accuracy is required for the detection of Ed compared with that in the current-voltage characteristic of FIG. 2.
FIG. 4 is a block diagram of a conventional shutdown control circuit which acts as a shutdown controlling means for a constant-current supply 21. The shutdown controlling means comprises a current detecting circuit 22, a voltage detecting circuit 23 and an OR gate 24. The current detecting circuit 22 detects the current fed by the constant-current supply 21 to a load resistance R, and outputs a signal when the current exceeds the value Ig, the shutdown current. The voltage detecting circuit 23 detects the voltage fed by the constant-current supply 21 and outputs a signal when the voltage exceeds the shutdown voltage Ed. The OR gate 24 outputs a control signal to cause the constant-current supply 21 to be shut down upon receiving an output signal from either the current detecting circuit 22 or the voltage detecting circuit 23.
Referring again to FIG. 3, the withstand voltage requirements can be eased further by providing the constant-current supplies with a steeper or inverted drooping characteristic as shown by the dotted lines including one connecting the drooping point "c" and the point Ee2 on the voltage axis and the other connecting the point "c" and Ee3 on the voltage axis. However, the respective shutdown points (not shown) on these drooping characteristic curves are at a voltage which is less than or equal to the voltage at the points "b2" and "b3" which are the respective intersections of the steeper and inverted drooping characteristic curves, and the load characteristic curve for the above-mentioned pseudo single-end feeding mode. Accordingly, the current supply is shut down as soon as the output voltage thereof reaches the voltage corresponding to the point "b2" or as soon as the output voltage reaches the voltage corresponding to the point "b3". This means that the withstand voltage reduction by the steeper or inverted drooping characteristic cannot be achieved by using the conventional shutdown circuit shown in FIG. 4. Therefore, there is a need for a novel controlling means for shutting down an electric power supply having the above-mentioned steep or inverted drooping characteristic.
The above description also applies to a constant-voltage supply if the relationship between the current and voltage in the current-voltage characteristic diagram of the constant-current supply are substituted for each other; however, the details will be discussed in the description of the preferred embodiments.